(1) Field of the Invention
The present invention relates to power systems in general and more particularly to circuitry for protecting said power systems from electrical faults.
(2) Prior Art
The use of power systems for driving various types of machine or loads is well known in the prior art. Of the various type prior art power systems, the switching regulator type power system is well suited for driving different types of industrial loads, machines, etc. The switching power system usually includes a switching transistor whose on-time cycle is controlled so that adequate power is delivered to the output load. Generally, the output load is coupled to the switching transistor through a power transformer.
Most power systems, be it of the switching regulator type or otherwise, are susceptible to damages due to electrical faults. The electrical faults may include a short circuit condition, an undervoltage condition, an overload condition, etc. Also, the electrical fault may occur in the power system and/or in the load. The utilization of protective circuitry as a means of protecting power systems from electrical faults is well known in the prior art. Various types of protective circuitry have been prepared and have been used to protect power systems.
A frequently used conventional technique for protecting power systems is that the power system is forced into operating at a low duty cycle (relaxation mode)when an electrical fault such as an overload occurs. In this mode of operation the power system outputs short bursts of current at a low repetition rate. Although the average output power drops to a relatively low level, the peak output current may be quite high. Usually, this method of protection works well in power supply whose rating is less than 30 watts. However, when the technique is used to protect a higher rating power supply, two problems arise. First, the high peak output may cause damage to the load even though the average power is low. Second, for a switch mode power supply, the power dissipation in the switching transistor base drive resistor becomes unacceptable. U.S. Pat. No. 4,447,841 is an example of the prior art wherein the mode of operation is switched when a fault condition occurs.
In order to provide for high power applications the base drive resistor that is usually present in the base lead of the switching transistor is replaced with an inductive device. Utilization of the inductive base drive technique is shown and described in an article entitled "Driver Circuit for Switching Regulator," by J. K. Radcliffe in the IBM Technical Disclosure Bulletin (Vol. 24, No. 11A, April 1972, pages 5501-5503). Although the inductive base drive technique provides an improvement over the prior art, it does not work well if a series capacitor is used with the inductor. As a result, the relaxation mode technique is not a suitable protection method for a higher rating power supply.
Current limiting is another conventional technique used in protecting power supplies. In this technique a relatively low value resistor is placed in the emitter lead of the switching transistor and circuitry is added to monitor the resistor voltage. Whenever the resistor voltage reaches a predetermined value, the switching transistor is turned off. Thus, a limit is placed on the peak current through the switching transistor by choosing the right resistor.
The general problem with the current limiting technique is that during "start-up" and/or "turn-off" the "on-time" of the switching transistor will be determined by the current limiting circuit. If the power supply is overloaded, etc., the current limiting circuit becomes effective. This provides protection for the switching transistor but it allows the entire output power capability of the supply to be delivered to the load. Some form of current limiting is practiced in U.S. Pat. Nos. 4,020,395; 4,156,273 and 4,428,015.
Still other types of protection schemes are given in U.S. Pat. Nos. 4,330,816; 3,931,567; 3,987,342 and 4,024,437. In these circuits different parameters are measured and if the measurement falls outside of a desired range, adjustment is made by controlling the switching transistor.